ASP.NET Core provides a way for us to create custom validations for our requests by extending ValidationAttribute, which means we can create custom validators for our types. Let's create a more appropriate validation for our Currency attribute:

using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Linq;

namespace SampleAPI.Requests
{
    public class CurrencyAttribute : ValidationAttribute
    {
        private readonly IList<string> _acceptedCurrencyCodes = 
        new List<string>{
            "EUR",
            "USD",
            "GBP"
        };

        protected override ValidationResult IsValid(object value, 
        ValidationContext validationContext)
        {
            return _acceptedCurrencyCodes.Any(c => c == value.ToString()) ?
                ValidationResult.Success 
                : new ValidationResult($"{validationContext.MemberName} is 
                not an accepted currency");
        }
    }
}

The preceding implementation matches the request model currency with the list of _acceptedCurrencyCodes. If the match is successful, it returns ValidationResult.Success; otherwise, it returns a new validation result with a validation message. The MemberName attribute provides the name of the property that is associated with the custom validation attribute. Creating a custom validation attribute can be useful when we implement a more complex validation that involves third-party services or aggregate operations on the subject of the validation.